The human epidermal growth factor receptor 2/c-erbB-2 (HER-2/neu) gene is localized to chromosome 17q and it encodes a transmembrane tyrosine kinase receptor protein that is a member of the epidermal growth factor receptor family (Coussens et al., 1985, Science 230:1132). The role of Her-2/neu is best characterized in breast cancer progression, in which amplification of the Her-2/neu gene was found to correlate with tumor over expression of Her-2/neu protein and be a significant predictor of time to disease recurrence and overall survival (Ross et al., 2004, Mol Cell Proteomics 3:379). In addition, data from breast cancer clinical trials indicate that the response to anti-Her-2/neu agents was largely limited to patients with the highest Her-2/neu over expression (Cobleigh et al., 1999, J Clin Oncol. 17:2639). As a result, diagnostic tests that characterize Her-2/neu status in patients with breast cancer have been approved by the Food and Drug Administration and incorporated into the standard of care (Ross et al., 2004, Mol Cell Proteomics 3:379). Studies indicate that Her-2/neu gene amplification is a less common event in prostate cancer than it is in breast cancer (Vernimmen et al., 2003, Br J Cancer 89:899). Nevertheless, to various degrees Her-2/neu protein over expression has been demonstrated in prostate cancer tumor tissue (Shi et al., 2001, J Urol. 166:1514; Osman et al., 2001, Clin Cancer Res. 7:2643, 2001). In primary untreated disease Her-2/neu over expression is uncommon. With the administration of neoadjuvant hormone therapy expression increases significantly (Shi et al., 2001, J Urol. 166:1514; Osman et al., 2001, Clin Cancer Res. 7:2643, 2001). The highest rates of over expression observed are in tumor tissues in patients with metastatic androgen independent disease, suggesting that treatment with agents that target Her-2/neu would be most appropriate in this subset of patients. Nevertheless, no firm conclusions regarding the efficacy of the anti-Her2/neu monoclonal antibody trastuzumab in patients with metastatic androgen independent prostate cancer could be drawn in the 2 clinical trials that have been attempted (Morris et al., 2002, Cancer 94:980). Each was limited by the technical difficulties observed when sampling metastatic tissue, which is often located in bone, and in accruing an adequate number of Her-2/neu positive patients. The experience gained from these investigations highlights the need for more practical screening methods to identify patients who may be candidates for the Her-2/neu inhibitor trastuzumab as well as for the various other agents that target the Her-2/neu signaling cascade (Morris et al., 2002, Cancer 94:980). The present invention is directed to measuring Her-2/neu levels in fluid biological samples of patients with prostate cancer as a predictor of disease progression, prognosis, and treatment indications.